docs/sched/sched: Add documentation for different sleep interfaces

As pull request apache#17200 & apache#17368 introduced support for scheduling sleep, a documentation is needed for different sleep interfaces.
This patch adds the description for sleep interfaces currently provided in NuttX, including Scheduled sleep(nxsched_sleep()...), Signal-scheduled sleep(nxsig_sleep()...), and Busy sleep(up_udelay()).

Signed-off-by: Haokun Dong <donghaokun@lixiang.com>

sched/sched/sched_sleep.c

@@ -226,14 +226,16 @@ void nxsched_sleep(unsigned int sec)
  * Name: nxsched_nanosleep
  *
  * Description:
- *   Internal nanosleep implementation used by the scheduler.  This function
+ *   Internal nanosleep implementation used by the scheduler. This function
  *   converts the requested sleep interval into system ticks, performs a
  *   tick-based blocking sleep, and optionally returns the remaining time if
  *   the sleep is interrupted by a signal.
  *
  * Input Parameters:
  *   rqtp - Requested sleep interval (may be NULL)
- *   rmtp - Remaining time returned when interrupted (optional, may be NULL)
+ *   rmtp - If the rmtp argument is non-NULL, the timespec structure
+ *          referenced by it is updated to contain the amount of time
+ *          remaining.
  *
  * Returned Value:
  *   Returns OK (0) on success.  Returns -EINVAL for an invalid timespec